Field of the Invention
This invention relates to self-aligned bipolar transistors and particularly to the structure and fabrication technique of a self-aligned bipolar transistor with an inverted polycide extrinsic base contact.
In a transistor integrated circuit, individual transistor devices are defined by relatively small differences of structure due to different doping materials which alter the conductivity for related polarities of electrical potentional, and by insulation layers grown as oxides or otherwise introduced. All items forming the individual devices are closely related in physical proximity, integrated into a single integrated circuit chip. An individual device may have emitter, base and collector, and be defined by relatively inactive surrounding material, all formed from the same silicon material through doping processes or the lack of doping. The active elements of a planar transistor may all be embedded within the silicon chip, and may be remarkably tiny. The emitter may be a small spot on the surface, extending for only a small distance into the bulk of the chip. The base may be even thinner, and be essentially hidden below the emitter, within the bulk of the chip. The collector may underlie the base and be completely submerged under the emitter and base, but extend outward so as to present itself for electrical contact at the surface. The base may have two regions of interest, the active base region (intrinsic base) under the emitter, and the inactive base region (extrinsic base) near the base contact.
Those skilled in the art of transistor integrated circuits understand the active function of the intrinsic base as contrasted to the conductivity function of the extrinsic base. "All the base current must flow through the inactive region, and the current path is through material of finite cnductivity. This portion of the path can be simulated by a fixed resistor in series with the base lead. It is called the `extrinsic` base resistance because it is outside the active region . . . Over a considerable range of current it is possible to represent the effect by an `intrinsic` base resistance added to the extrinsic base resistance." Paul E. Gray, David DeWitt, A. R. Boothroya and James F. Gibbons, Physical Electronics and Circuit Models of Transistors, John Wiley & Sons, New York,
Refractory metals or silicides of refractory metals have been used as base contacts to which the emitter of a bipolar transistor is self-aligned. This reduces the extrinsic base area and the series resistance of the extrinsic base of the bipolar transistor. T. H. Ning and H. N. Yu, "Bipolar Transistor Structure with Extended Metal Base Contacts and Diffused or Implanted Emitter," IBM Technical Disclosure Bulletin, Vol. 22, No. 5, October 1979, pages 2123-2126. However, this structure does not allow independent doping of the extrinsic and intrinsic base regions.
An alternative process uses p.sup.+ polysilicon as both a base contact and a diffusion source for the extrinsic base. U.S. Pat. No. 4,157,269, T. H. Ning and H. N. Yu, "Utilizing Polysilicon Diffusion Sources and Special Masking Techniques," June 5, 1979. The sheet resistivity of p.sup.+ polysilicon is considerably higher (.about. 100.OMEGA./square) than that of silicides (.about.3.OMEGA./square). A diffusion doping technique for forming intrinsic base and extrinsic base regions in transistor integrated circuits is described at Column 2, Lines 60-65. Those skilled in the art have come to refer to a stack of successive layers of silicon dioxide, doped polysilicon and a metal silicide as a "polycide" stack, and to refer to polysilicon as "polysi." The function of the metal silicide in the polycide stack is to lower the resistance. The function of the polysi is to insulate the silicide from possibly reactive materials and gases. The function of the silicon dioxide is passivation and insulation.
The conventional polycide stack of doped polysilicon with a layer of silicide on top is difficult to pattern with either reactive ion etching or preferential chemical etchants in the fabrication of bipolar transistor structures where the polycide is etched down to silicon as opposed to the fabrication of FET structures where the polycide is etched down to SiO.sub.2.